The invention relates generally to luminescent materials and, more particularly, to scintillator compositions which are especially useful for detecting gamma-rays and X-rays under a variety of conditions.
Scintillators can be used to detect high-energy radiation, in processes, which are both very simple and very accurate. The scintillator materials are in common use as a component of radiation detectors for gamma-rays, X-rays, cosmic rays, and particles characterized by an energy level of greater than about 1 keV. The scintillator crystal is coupled with a light-detection means, i.e., a photodetector. When photons from a radionuclide source impact the crystal, the crystal emits light. The photodetector produces an electrical signal proportional to the number of light pulses received, and to their intensity.
The scintillators have been found to be useful for applications in chemistry, physics, geology, and medicine. Specific examples of the applications include positron emission tomography (PET) devices; well-logging for the oil and gas industry, and various digital imaging applications. Scintillators are also being investigated for use in detectors for security devices, e.g., detectors for radiation sources, which may indicate the presence of radioactive materials in cargo containers.
The composition of the scintillator is critical to device performance in all of these applications. The scintillator must be responsive to X-ray and gamma ray excitation. Moreover, the scintillator should possess a number of characteristics, which enhance radiation detection. For example, most scintillator materials must possess high light output, short decay time, high “stopping power”, and acceptable energy resolution. (Other properties can also be very significant, depending on how the scintillator is used, as mentioned below).
Various scintillator materials, which possess most or all of these properties have been in use over the years. Examples include thallium-activated sodium iodide (NaI (Tl)); bismuth germanate (BGO); cerium-doped gadolinium orthosilicate (GSO); cerium-doped lutetium orthosilicate (LSO); and cerium-activated lanthanide-halide compounds. Each of these materials has properties, which are very suitable for certain applications. However, many of them also have some drawbacks. The common problems are low light yield, physical weakness, and the inability to produce large-size, high quality single crystals. Other drawbacks are also present. For example, the thallium-activated materials are very hygroscopic, and can also produce a large and persistent after-glow, which can interfere with scintillator function. Moreover, the BGO materials frequently have a slow decay time. On the other hand, the LSO materials are expensive, and may also contain radioactive lutetium isotopes, which can also interfere with scintillator function.
In general, those interested in obtaining the optimum scintillator composition for a radiation detector have been able to review the various attributes set forth above, and thereby select the best composition for a particular device. (As but one example, scintillator compositions for well-logging applications must be able to function at high temperatures, while scintillators for PET devices must often exhibit high stopping power). However, the required overall performance level for most scintillators continues to rise with the increasing sophistication and diversity of all radiation detectors.
As an example, in well-logging applications, scintillator crystals must be able to function at very high temperatures, as well as under harsh shock and vibration conditions. The scintillator material should therefore have a maximized combination of many of the properties discussed previously, e.g., high light output and energy resolution. (The scintillator must also be small enough to be enclosed in a package suitable for a very constrained space). The threshold of acceptable properties has been raised considerably as drilling is undertaken at much greater depths. For example, the ability of conventional scintillator crystals to produce strong light output with high resolution can be seriously imperiled as drilling depth is increased.
It should thus be apparent that new scintillator materials would be of considerable interest, if they could satisfy the ever-increasing demands for commercial and industrial use. The materials should exhibit excellent light output. They should also possess one or more other desirable characteristics, such as relatively fast decay times and good energy resolution characteristics, especially in the case of gamma rays. Furthermore, they should be capable of being produced efficiently, at reasonable cost and acceptable crystal size.
Accordingly, a need exists for an improved scintillator material that may address one or more of the problems set forth above.